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<TITLE>Implicit Synchronization by Tying Streams</TITLE>
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<H2>35.4 Implicit Synchronization by Tying Streams</H2>
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<P>Another mechanism for automatic synchronization in certain cases is tying a stream to an output stream, as demonstrated in the code below. All input or output operations flush the tied stream's buffer before they perform the actual operation.</P>

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std::ofstream ostr("/tmp/fil");
std::ifstream istr("/tmp/fil");
std::ostream* old_tie = istr.tie(&amp;ostr);                      //1

while (some_condition) {
  ostr &lt;&lt; "... some output ...";
  std::string s;
  while (istr &gt;&gt; s)                                           //2
     // process input ;
}
istr.tie(old_tie);                                            //3
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<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>The input stream <SAMP>istr</SAMP> is tied to the output stream <SAMP>ostr</SAMP>. The <SAMP>tie()</SAMP> member function returns a pointer to the previously tied output stream, or <SAMP>0</SAMP> if no output stream is tied.
<TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>Before any input is done, the tied output stream's buffer is flushed so that the result of previous output operations to <SAMP>ostr</SAMP> is available in the external file <SAMP>/tmp/fil</SAMP>.
<TR VALIGN="top"><TD><SAMP>//3</SAMP></TD><TD>The previous tie is restored.
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